Current collector for solid oxide fuel cell tube with internal fuel processing

ABSTRACT

A solid oxide fuel cell includes a tube, a spacer element, a catalytic substrate and an anode current collector. The solid oxide fuel cell further includes a spacer element disposed within the tube. The solid oxide fuel cell further includes a catalytic substrate disposed within the anode current collector electrically contacting the anode of the tube and providing an electrical current path inside the tube past the catalytic substrate to the inlet opening.

RELATED APPLICATION

This application is a continuation in part of patent application10979017, which claims priority benefit of U.S. provisional patentapplication No. 60/515,779 filed on Oct. 30, 2003.

GOVERNMENT INTERESTS

This invention was made with government support under contract numberDAAD19-01-C-0073, awarded by the U.S. Department of Defense. Thegovernment has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to solid oxide fuel cells and more particularly,to solid oxide fuel cells of lightweight design.

BACKGROUND OF THE INVENTION

A solid oxide fuel cell (SOFCs) is a type of fuel cell which reacts afuel gas with an oxidant to generate DC electric current. SOFCscomprises an anode, an electrolyte and a cathode, and have been madefrom a variety of materials and in a variety of geometries. Fuelprocessing is required to render hydrocarbon fuels (such as propane,butane, etc.) suitable for SOFCs. For example, known designs for fuelprocessors include those done with a separate external reactor where acatalytic substrate processes a hydrocarbon fuel such as butane (C₄H₁₀),propane (C₃Hs) or diesel fuel (JP-8 or JET-A) to a suitable fuel gassuch containing carbon monoxide (CO) and hydrogen (H₂). CO and Hydrogengas are then oxidized at an active area of a SOFC to carbon dioxide andwater, with DC current generated. Non hydrocarbon fuels such as ammonia(NH₃) can also be transformed into SOFC fuel using one or more catalyticreactions.

It has become desirable to make solid oxide fuel cells as light aspossible so that they may serve as a portable power source. An externalreactor is bulky and is accompanied with significant inefficiencies intransfer of hot processed fuel gas from the reactor to the active areaof the fuel cell. U.S. Patent Publication 10 2003/0054215 to Doshi et aldiscloses a stack of fuel cell plates where a catalytic substrate ispositioned within the plates and within a thermal enclosure. However,such stack designs are known to have problems with thermal cycling, theycannot be heated and cooled quickly, and extensive seals have to beused, including seals subjected to high thermal loading. Further, hotseals have to be used at locations inside the thermal enclosure, and hotseals are expensive. Addressing all of these problems makes such stackedplate designs relatively expensive. It would be desirable to provide asolid oxide fuel cell of a simple, and lightweight design which is alsorobust in construction and capable of withstanding thermal cycling.

SUMMARY OF THE INVENTION

In accordance with exemplary embodiment, a solid oxide fuel cellincludes a tube, a spacer element, a catalytic substrate and an anodecurrent collector. The tube includes an inlet opening, an exhaustopening, an electrolyte, an anode positioned interiorly from theelectrolyte, and a cathode positioned exteriorly from the electrolyte.The solid oxide fuel cell further includes a spacer element disposedwithin the tube. The solid oxide fuel cell further includes a catalyticsubstrate disposed within the spacer element, the catalytic substratebeing configured to receive unreformed fuel and to convert theunreformed fuel to reformed fuel. The solid oxide fuel cell stillfurther includes an anode current collector contacting the anode of thetube and providing an electrical current path inside the tube past thecatalytic substrate to the inlet opening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a solid oxide fuel cell with internalprocessing in accordance with a preferred embodiment.

FIG. 2 is a schematic cross section view of the solid oxide fuel cell ofFIG. 1.

FIG. 3 shows a manifold and a series of SOFC tubes, and the catalyticsubstrate would preferably be positioned in each tube near the left end.

FIG. 4 shows a schematic of a single tube with a catalytic substrate anda space element positioned between the SOFC tube and the catalyticsubstrate.

FIG. 5 is a side view of the catalytic substrate showing a honeycombcross-section.

FIG. 6 is an alternate preferred embodiment of a catalytic substrateshowing an internal opening for a current collector.

FIG. 7 depicts the catalytic substrate of FIG. 6 having a currentcollector disposed therethrough.

FIG. 8A. shows a plurality of tubes in accordance with an exemplaryembodiment.

FIGS. 8B and 8C show a plurality of tubes in accordance with alternateembodiments.

FIG. 9 shows a cross-section of a tube in accordance with an exemplaryembodiment.

FIG. 10 shows a manifold and a plurality of SOFC tubes in accordancewith an exemplary embodiment.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the solid fuel cell asdisclosed here, including, for example, specific dimensions of thecatalytic substrate will be determined in part by the particularintended application and use environment. Certain features of theillustrated embodiments have been enlarged or distorted relative toothers for visualization and clear understanding. In particular, thinfeatures may be thickened, for example, for clarity of illustration. Allreferences to direction and position, unless otherwise indicated, referto the orientation of the solid state electrochemical device illustratedin the drawings.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

It will be apparent to those skilled in the art, that is, to those whohave knowledge or experience in this area of technology, that many usesand design variations are possible for the fuel cells disclosed herein.The following detailed discussion of various alternative and preferredfeatures and embodiments will illustrate the general principles of theinvention with reference to an internal reformer suitable for use with asolid oxide fuel cell (“SOFC”). Other embodiments suitable for otherapplications will be apparent to those skilled in the art given thebenefit of this disclosure.

FIG. 1 is a schematic of a solid oxide fuel cell 10 and surroundingcomponentry in accordance with a preferred embodiment. The fuel cell 10generates power to run a device 30 or to provide power to a secondarydevice 28 (such as charging a battery). Control electronics 24 and powerelectronics 26 may be mounted to control the power generation of theSOFC. A fuel tank 20 contains a fuel (typically butane, propane, diesel,JET-A, etc.) and a fuel regulator or pump 22 delivers the fuel into athermal enclosure defined by insulation 12. The SOFC 10 generatessignificant heat during operation (on the order of 600-1000° C.) and ispreferably mounted within the thermal enclosure. The SOFC is mostefficient at these higher temperatures, and therefore several designfeatures have been incorporated to heat incoming fuel gas and incomingair. For example, an air pump 14 pumps air from a cold air inlet 36 to arecuperator 16. The recuperator is essentially˜heat exchanger whichtransfers some of the heat from exhaust to the incoming air supplied tothe cathode. Ambient air for fuel processing is supplied to mix with thefuel in a fuel air mixer 38 in a predetermined ratio, preferably havinga sub-stoichiometric quantity of oxygen so that processing of the fuelgas takes place at the catalytic substrate 32 as described in greaterdetail below. Processing is understood here to mean conversion of a fuelto a gas which can be used by the SOFC 10, typically containing carbonmonoxide and hydrogen gas. The heated air circulates along the activearea and participates in electrochemically transforming the fuel gasinto electricity and exhaust gases.

FIG. 2 shows a schematic of a cross section of the preferred embodimentof the SOFC of FIG. 1, showing the fuel air mixture being introducedinto the thermal enclosure through a series of tubes 40. The actualnumber of tubes depends in part on the desired power output of the SOFC.A plurality of tubes is preferably mounted on a manifold 45, as seen inFIG. 3. Each tube 40 has an interior 41 and an exterior 43 (shown inFIG. 4). Preferably an anode material is positioned on the interior, andan electrolyte 47 exteriorly surrounds the anode. The entire tube 40 mayhave an interior layer of anode and a layer of electrolytecircumferentially surrounding the anode, so that the anode is remotefrom the exterior 43 of the tube and the electrolyte is remote from theinterior 41 of the tube. A cathode 48 is positioned around the exterior43 of the tube 40, defining an active area 44, also preferably remotefrom the interior of the tube. Fuel gas flows from the manifold 45, thento the catalytic substrate, and then to the active area 44 within eachtube. The catalytic substrate may be positioned immediately preceding(in terms of fuel gas flow) the active area and within the thermalenclosure so that heat generated at the catalytic substrate helpspreheat the fuel gases. That is, air from air inlet 36 passes throughthe recuperator 16 and is heated by an exhaust stream of gases movingseparately through the recuperator. For further heating, the air travelsthrough an air inlet tube 58 past the active areas 44 of the fuel cellsto an area generally adjacent the catalytic substrates 32. Residualoxygen in the heated inlet air is used to completely combust anyresidual fuel in the exhaust stream at the burner region 97. Optionallya catalytic element may be positioned at the burner region to help withcombustion. Once the reaction is complete, exhaust gas is routed throughexhaust tube 60, through the recuperator 16, and out of the thermalenclosure. Thus, the major inlets and outlets to the thermal enclosureare air inlet 36, fuel gas inlet 38, and exhaust gas outlet 60. Suchfuel cell designs are advantageously relatively light in weight, andprovide good power density to mass ratios. As an example of alightweight design each tube 40 can comprise a 1 mm-20 mm diameter tubewith at least three layers, an inner layer of anode, a middle layer ofelectrolyte, and an outer layer of cathode. The anode comprises, forexample, a porous cermet of nickel and yttria stabilized zirconia (YSZ).The electrolyte can comprise a thin membrane of YSZ. The cathode cancomprise, for example, a porous lanthanum strontium manganate (LSM). Anexample of a suitable fuel cell tube shaped anode, electrolyte andcathode is disclosed in U.S. Pat. No. 6,749,799 to Crumm et al, entitledMETHOD FOR PREPARATION OF SOLID STATE ELECTROCHEMICAL DEVICE and herebyincorporated by reference. Other material combinations for the anode,electrolyte and cathode, as well as other cross section geometries(triangular, square, polygonal, etc.) will be readily apparent to thoseskilled in the art given the benefit of this disclosure. In accordancewith a highly advantageous feature, the catalytic substrate 32, whichpartially oxidizes the fuel gas, is positioned inside the tube,advantageously eliminating the need and expense of an externalprocessing device. Moreover, significant heat is generated at thecatalytic substrate 32 when a fuel gas is introduced, and this heat isuseful in increasing the temperature of the gases to a level where thefuel cell is more efficient in generating electrical power.

The catalytic substrate 32 can comprise, for example, particles of asuitable metal such as platinum or other noble metals such as palladium,rhodium, iridium, osmium, or their alloys. Other suitable materials foruse as a catalytic substrate can comprise oxides, carbides, andnitrides. The catalytic substrate may comprise a wire, a porous bulkinsert of a catalytically active material, or a thin “ribbon” whichwould have the effect of increasing the ratio of surface area to volume.In the preferred embodiment shown in FIG. 5, the catalytic substrate 32comprises a supported catalyst. Supported catalysts consist of very finescale catalyst particles supported on a substrate made of, for example aceramic such as aluminum oxide. Preferably the catalytic substrate ishoneycomb shaped, provided with a series of openings 68 which the fuelgas passes through as it is processed. The outer diameter of thehoneycomb catalytic substrate is smaller than the inner diameter of theof SOFC tube 40. The honeycombed catalytic substrate may also beprovided with spaces through which any current collector wires may pass.Other materials suitable for use as a catalytic substrate will bereadily apparent to those skilled in the art given the benefit of thisdisclosure.

The catalytic substrate 32 is shown in FIGS. 1-2 positioned inside thethermal enclosure defined by the insulation 12. FIG. 3 shows theinsulation 12 removed showing tubes 40 of the SOFC bundled together bymanifold 45. In this manner, a series of tubes can be assembled togetherinto a close fitting compartment surrounded by insulation 12. In thismanner, cold hydrocarbon fuel and oxygen advantageously can beintroduced to the SOFC through tubes closed off at the manifold withconventional or “cold seals” such as adhesives and sealants.

Current collectors may be mounted within and around the tubes 40,preferably at or near the active area 44 to capture electric currentgenerated when the fuel gases are completely oxidized. FIG. 6 shows analternate preferred embodiment of a tube-shaped catalytic substrate 132where an opening 61 may be provided to receive a current collector.Alternatively, a packed bed of catalytic substrate beads may be used,with the beads either arranged in an orderly fashion or randomly packed.

FIG. 4 shows a single tube 40. Extending along the entire length of thetube 40 along its interior 43 is the anode, and extendingcircumferentially around the anode is the electrolyte. Advantageously,the cathode need only be positioned at the active area 44, shown in FIG.4 positioned circumferentially around at least part of the tube. Thecatalytic substrate 32 is positioned within the tube and spaced apartfrom the active area. In accordance with a highly advantageous feature,a spacer element 46 is provided which physically isolates the catalyticsubstrate 32 from the tube 40. This is advantageous in that when fuelpasses through the catalytic substrate, significant heat is generated.Spacer element helps thermally isolate the tube, thereby reducing heatshock and related stresses. In a preferred embodiment the spacer elementhas a fixed end and a free end, and is mounted to the tube 40 at thefixed end. The catalytic substrate is positioned near the free end. Thespacer element 46 can comprise, for example, a metal such as stainlesssteel or a ceramic such as zirconia or alumina. Other compositions forthe spacer element will be readily apparent to those skilled in the artgiven the benefit of this disclosure.

Advantageously, the internal catalytic substrates can be combined withdifferent compositions in series to effect controlled ignition, thermaldistribution and reaction, as desired. A fuel mixture consisting of oneor more of water vapor, fuel, and oxygen can first pass over a catalyticsubstrate which partially oxidizes a portion of the fuel to create a hotmixture of the remaining fuel, and one or more of partially reformedfuel, hydrogen, carbon dioxide, carbon monoxide, and water. The heatedfuel-water from the partial oxidation reaction could then undergoendothermic steam reforming in a downstream internal reactor or acrossthe same reactor (tube) to produce a desired reformed fuel.

This type of fuel processing is known as autothermal reforming.Alternatively, a suitable mixture of fuel and water vapor used incombination with one or more catalytic substrates may be used totransform a hydrocarbon into a desired reformed fuel. Additionalcatalytic materials may be applied to promote a ‘water gas shift’reaction which would enrich the hydrogen content of a fuel gas stream.

FIG. 7 shows the substrate 132 with the opening 61 receiving a currentcollector 70, therethrough. FIG. 8A shows multiple tubes (tubes 40)comprising an anode 49, the electrolyte 47, and the cathode 48 disposedon a portion of the electrolyte 47 defining an active area 44. A cathodecurrent collector is wrapped around an exterior of the tube 80 a lengthof the active area 44. FIG. 8B shows multiple tubes (tubes 140) in whichis a current collector 170 electrically connects the tubes 140 inseparate bundles in a parallel connection. In one embodiment, electricalcurrent being supplied to each bundle of cells can be controlledseparately by utilizing a diode, switch or other electric circuitcomponent. FIG. 8C shows multiple tubes (240) in which a currentcollector 170 electrically connects the tubes 240 in a series connectionsuch that an anode of a first fuel cell tube is connected to a cathodeof a second fuel cell tube.

FIG. 9 shows a cross-section of the tube 40. The anode current collector70 includes an elongated portion 72 and a brush portion 74. Theelongated portion 72 is relatively narrow in diameter and extendsthrough the catalytic substrate 132 and an inner length of the spacerelement 46.

The exemplary brush portion 74 comprises a wire brush having an innercore 101 and a plurality of loop members 102 extending therefrom. Theinner core 101 functions as an arterial electrical conduit providingcurrent conduction the length of the brush portion 74. The loop membershave resilient properties and overall diameter of the anode currentcollector 74 can be set so that the loop members 102 are compressedagainst an inner wall of the fuel cell tube 40 proximate the activeportion 44. Continuous lengths of compressed loop members 102 contactthe anode 49 of the fuel cell tube 18 to promote electrical contact withthe anode 49. The anode current collector 70 remains mechanicallycomplaint allowing the anode current collector 74 to distribute forcescreated by mechanical and thermal expansion stresses so that the brushportion 74 maintains contact with the anode layer 49.

The loop members route electrons relatively short distances between theanode layer 49 and the inner core 101. The loop members 102 have spacetherebetween to allow fuel to pass between the loop members. In oneembodiment, the loop members provide a calibrated cross section toprovide a selected amount of backpressure. The anode current collector74 comprises an electrically conductive metal. Since the anode currentcollector 74 brush members is positioned in fuel gas and in processedfuel gas, the anode current collector 74 is formed from material thatmaintains conductivity in the operating environment of the inner chamberof the fuel cell tube 40. In alternate embodiments, other currentcollector designs can be utilized including current collectors with abrush member comprising bristle members or other shapes contacting theanode 48. Exemplary materials for the anode current collector includestainless steel, copper and copper alloys, silver and silver alloys, andnickel and nickel alloys.

The tubes 40 have the current collectors 70 extending from active areas40 of tubes 80 through the substrate 132, through the spacer elements 46and exiting a fuel inlet (‘FUEL’) of the spacer element 46. Anodes ofindividual tubes can be electrically connected to each other by thecurrent collector 70.

FIG. 10 shows an exploded prospective view of the fuel cell 10. Themanifold 45 includes a faceplate 39 comprising an air and fuel inlet 37,a ring member 41, and a base portion 43. The faceplate 39 can form agas-tight seal with the base portion 43 with the ring member disposedtherebetween such that fuel can routed between from the fuel inlet 37 toa gas-tight inner chamber defined by the face plate 39 and the baseportion 43. The term gas-tight as used herein is used to refersubstantially preventing fluid flow circumferentially out of the innerchamber, thus the inner chamber routes substantially all fluid from theair and fuel inlet 37 through the inner chamber to the plurality ofspacer 46. The base portion 43 further includes connection members 47 tofurther provide a seal between the inner chamber and the spacers 46. Thefuel cell system 10 further includes a support member 41 having tubes 40routed therethrough.

During assembly, the elongated portion 74 of the current collectors 40can be disposed though the opening of the catalytic substrate 132 insidethe spacer elements, routed through the interior of the spacer elements,and electrically connected to each other at the manifold. The anodecurrent collector 70 and the spacer members 46 can be inserted into thetubes 40 and the spacer members 46 can be sealed to fuel cell tubeutilizing a sealant, for example a ceramic sealant or a silicon basedsealant, (not shown) disposed around an exterior spacer members 46.

The catalytic substrates disclosed herein are advantageous in that theymay be used, in any of the aforementioned processes where a catalyticagent is required to accelerate a reaction. From the foregoingdisclosure and detailed description of certain preferred embodiments, itwill be apparent that various modifications, additions and otheralternative embodiments are possible without departing from the truescope and spirit of the invention. The embodiments discussed were chosenand described to provide the best illustration of the principles of theinvention and its practical application to thereby enable one ofordinary skill in the art to use the invention in various embodimentsand with various modifications as are suited to the particular usecontemplated. All such modifications and variations are within the scopeof the invention as determined by the appended claims when interpretedin accordance with the breadth to which they are fairly, legally, andequitably entitled.

1. A solid oxide fuel cell comprising: a tube comprising an inletopening, an exhaust opening, an electrolyte, an anode positionedinteriorly from the electrolyte, and a cathode positioned exteriorlyfrom the electrolyte; a spacer element disposed within the tube; acatalytic substrate disposed within the spacer element, the catalyticsubstrate being configured to receive unreformed fuel and to convert theunreformed fuel to reformed fuel; and an anode current collectorcontacting the anode of the tube and providing an electrical currentpath inside the tube past the catalytic substrate to the inlet opening.2. The solid oxide fuel cell of claim 1, wherein the catalytic substratedefines an opening and wherein the anode current collector is receivedthrough the opening of the catalytic substrate.
 3. The fuel cell systemof claim 1, wherein the inner electrode and the outer electrode definean active area for ion conduction and wherein the active area ispositioned in closer proximity to the exhaust opening of the tube thanthe inlet opening of the tube.
 4. The fuel cell system of claim 3,wherein the active area is disposed downstream from and proximate to thecatalytic substrate.
 5. The fuel cell system of claim 1, wherein thespacer element substantially prevents unreformed fuel from contactingnickel of the anode.
 6. The solid oxide fuel cell of claim 1, whereinthe catalytic substrate comprises a supported catalyst.
 7. The fuel cellsystem of claim 1, further comprising a manifold member defining aninner chamber, said manifold member being configured to route fluid tothe inlet end of the fuel cell tube, wherein the anode current collectorextends to the inner chamber of the manifold member.
 8. The fuel cellsystem of claim 1, wherein anode current collector comprises a brushportion.
 9. The fuel cell system of claim 9, wherein the brush portioncomprises loop members contacting the anode of the tube.
 10. The fuelcell system of claim 1, wherein the catalytic substrate is disposedproximate to the active area of the tube.
 11. A solid oxide fuel cellcomprising: a plurality of tubes, each comprising an inlet opening, anexhaust opening, an electrolyte, an anode positioned interiorly from theelectrolyte, and a cathode positioned exteriorly from the electrolyte; aspacer element disposed within each tube; a catalytic substrate disposedwithin each spacer element, the catalytic substrate being configured toreceive unreformed fuel and to convert the unreformed fuel to reformedfuel; and an anode current collector contacting the anode of each tubeand providing an electrical current path inside the tube past thecatalytic substrate to the inlet opening.
 12. The fuel cell system ofclaim 11, further comprising a manifold member having an inner chamberconfigured to route fluid to each of the plurality of tubes, whereineach anode current collector extends to the inner chamber of themanifold member and wherein a first tube is electrically connected to asecond tube by a wire extending through the manifold member.
 13. Thesolid oxide fuel cell of claim 11, wherein each catalytic substratedefines an opening and wherein each anode current collector is receivedthrough the opening.
 14. The fuel cell system of claim 11, wherein theinner electrode and the outer electrode of each tube define an activearea for ion conduction and wherein the active area is positioned incloser proximity to the exhaust opening of the tube than the inletopening of each tube.
 15. The fuel cell system of claim 15, wherein theactive area is disposed downstream from and proximate to the catalyticsubstrate.
 16. The fuel cell system of claim 11, wherein the spacerelement substantially prevents unreformed fuel from contacting nickel ofthe anode.
 17. The solid oxide fuel cell of claim 11, wherein thecatalytic substrate comprises a supported catalyst.
 18. The fuel cellsystem of claim 11, wherein anode current collector comprises a brushportion.
 19. The fuel cell system of claim 11, wherein the brush portioncomprises loop members contacting the anode of the tube.
 20. The fuelcell system of claim 1, wherein the catalytic substrate is disposedproximate to the active area of the tube.